The present invention relates generally to etch masks for microstructuring of glass materials and particularly, to photolithographically created durable chromium-based laminate etch masks for use in microstructuring anodically bondable glass materials or other glass materials used in sensor fabrication, such as quartz and fused silica.
Silicon and various types of glass are widely used in microsensor and microactuator technologies because they are formable into microstructures, commonly referred to as being “microstructurable,” and because they provide other advantageous properties, including: electrical insulation, resistance to many chemicals, transparency at some wavelengths, and joinability to many different metals. The most significant requirements for materials used in the microsystem technologies are that the glass be microstructurable, preferably using a standard photolithography batch process; contain easily movable ions for anodic bonding; and have a thermal expansion coefficient essentially matching the thermal expansion coefficient of silicon to avoid thermal stresses at the interface. Thus, the preferred materials for use in microsystem technologies are both easy to structure using standard photolithography batch process techniques and easy to anodically bond. Generally, however, the chemical composition of anodically bondable glass materials causes etching difficulties while easily microstructurable glass materials are generally difficult to bond because of thermal expansion coefficients which are not well matched to the thermal expansion coefficient of silicon and/or the absence of easily movable positive sodium ions for anodic bonding. One anodically bondable glass materials commonly used in microsensor and microactuator technology are borosilicate Pyrex™ 7740 glass available in wafers from Corning Inc. of Corning, N.Y. Other glass materials that are easy to structure using standard photolithography batch process techniques are commonly used in sensor fabrication. Such materials include quartz and fused silica, for example, high purity fused silica Corning™ 7890 glass wagers, also available from Corning Inc., are used in the manufacture of some structured glass sensors. some structured glass sensors.
One known method for structuring both anodically bondable glass materials and other structurable glass materials is electro-chemical discharge drilling which includes, for example, chemical-assisted laser etching and focused ion beam enhanced etching. Other known drilling methods use mechanical drills, ultrasonic drills, focused ion beam drills, both eximer and carbon dioxide laser drills, and micro-sandblasting. While these glass drilling methods obtain straight walls, these methods provide neither the accuracy desired in microsystems nor the desired batch processing capabilities. Rather, these methods are generally incur relatively high costs, both in terms of the equipment required in the processing and the process cycle time.
Batch processing methods for microstructuring are necessary to the practical use of both anodically bondable glass materials and other structurable glass materials used in sensor fabrication, such as quartz and fused silica. One common microstructuring batch process involves masking portions of the glass wafers and etching the desired details into those portions of glass exposed by the mask. Photolithography is commonly used to define a mask material resistant to the etchant. Thus photolithography provides a process whereby mask details are accurately registered on batch quantities of glass wafers. However, the known methods of photolithographically depositing mask materials are costly and, in the case of polysilicon deposition, extreme cleanliness is required for high quality machining. A major limitation of currently available masking techniques is undercutting of the mask due to poor adhesion to the glass. The undercutting causes an expansion of the mask details within the glass wafer which leads to poor resolution of the details.
An article by Corman, Enoksson and Stemme entitled Deep Wet Etching of Borosilicate Glass Using an Anodically Bonded Silicon Substrate as Mask, published in the Journal of Micromaching and Microengineering, Vol. 8 (1998) at pages 84-87 detailed a method of deep etching in borosilicate glass using an anodically bonded silicon substrate as a mask in a standard lithography technique. The method was developed using 500 micrometer thick borosilicate Pyrex™ 7740 glass wafers. The masking layer was a silicon wafer previously microstructured in a potassium hydroxide (KOH) solution and anodically bonded to the glass wafer. The substrates were then submerged in a solution of 50% hydrofluoric (HF) acid mixed with water in a 1:5 concentration. Although the method disclosed may be realizable using a standard lithography batch process, the method results in an undesirable lateral undercutting 1.5 times larger than the etch depth.
Another method for structuring both anodically bondable glass materials and other structurable glass materials uses a polysilicon film deposited by low pressure chemical vapor deposition or LPCVD on a 500 micrometer thick Pyrex™ glass wafer as detailed in an article by Grétillat, Thlébaud, Koudelka-Hep and de Roolj entitled A New Fabrication Method of Borosilicate Glass Capillary Tubes With Lateral Inlets and Outlets, published in the Proceedings of Eurosensors (1996) at pages 259-62. This polysilicon film method is reportedly compatible with standard lithography techniques but, during the high temperature LPCVD deposition process, the Na+ ions diffuse out of the glass and contaminate the furnace. This contamination seriously degrades the electrical characteristics of silicon so that the costly furnace is only usable thereafter for “unclean” applications. A similar method using plasma enhanced chemical vapor deposition or PECVD silicon carbide as a mask was reported by Flannery, Muurlos, Storment, Tsai, Tan, and Kovacs in an article entitled PECVD Silicon Carbide for Micromachined Transducers, in the proceedings of Transducers '97 at pages 217-20.
Other known methods for structuring glass materials use etch masks formed of chromium, gold and resist wherein the chromium layer improves adhesion to the glass substrate. Although simple when compared with standard lithography techniques, these methods fail when applied to etch depths over about 50 micrometers because pin holes develop with prolonged exposure to the hydrofluoric acid etchant. Moreover, a substantial lateral under etching of chromium occurs caused by a high etch rate at the glass-chromium interface. If the process is continued, this lateral undercutting results in complete separation of the mask from the glass wafer. While this lateral undercutting can be reduced by heating the glass substrate before and after the chromium mask deposition to diffuse some of the chromium into the substrate, adhesion to the glass substrate remains poor and the mask exhibits poor resistance to the etchant. The poor adhesion and poor resistance to HF generally limits use of this type of mask to shallow etchings of 50 micrometers or less.
Thus, a need exists for a process of microstructuring anodically bondable glass materials and other structurable glass materials realizable in a low cost batch process and yielding accurately located, high resolution, high aspect ratio features.